Temporal and Petrogenetic Constraints on Volcanic Accretionary Processes at 9-10 Degrees North East Pacific Rise

Major and trace element, and Sr, Nd, Hf, Pb, and 238U-230Th-226Ra isotope compositions were measured in mid-ocean ridge basalts (MORB) from 9º-10ºN along the East Pacific Rise (EPR) to determine eruption ages and to place temporal constraints on volcanic and magmatic processes. At 9º30’N EPR, 238U-230Th-226Ra compositions indicate enriched MORB (E-MORB) collected off-axis erupted >8 ka and that E-MORB magmatism is interspersed with normal MORB magmatism. Lava ages are consistent with eruption from the axial summit trough (AST) and flow down the ridge flanks, which is in contrast to previous studies that suggested E-MORB erupted from off-axis vents. At 9º50’N EPR, seven discrete eruptive units are distinguished out to ~0.8-2 km on either side of the AST by high precision 238U, 232Th, and 226Ra sample concentrations. Model ages indicate the neo-volcanic zone is <<2 ka and is generated more rapidly than previously recognized. (226Ra/230Th) disequilibria measured in east limb lavas from the 9º03’N overlapping spreading center indicate recent basaltic volcanism (<<8 ka) occurring up to ~4 km off-axis. The axial graben sources the most recent volcanic activity including eruption of dacites. Major and trace element, 87Sr/86Sr, and (234U/238U) isotope compositions are consistent with the formation of dacites by extensive crystallization.